Many buildings, particularly industrial and high-rise buildings are constructed by erecting a steel girder framework with the above-ground floors consisting of steel decking supported by the beams of the girder framework and the decking itself supporting a concrete floor. The floor spans are limited by the bending stresses in the decking due to the weight of the concrete floor, and the deflection of the decking and concrete floor. In order to increase the floor span, it is known to prop the decking at mid-span until the concrete floor has set and reached adequate strength. However, this strength achieving time can be of the order of four weeks, and meanwhile the presence of the props restricts further construction activity. In addition, the props are costly and there is the additional time and cost of fitting and removal. Alternatively, the decking may be supported by means of additional “secondary beams” secured to the beams of the girder framework, but again these are an additional expense. Furthermore, the presence of the secondary beams restricts the passage of services, e.g. gas, water and electricity pipes and cables, through the floor space. As a further alternative, the flooring may be formed of pre-stressed concrete, but this is very costly to produce and transport to the site. In addition, large capacity lifting gear is required to position the flooring.
To avoid or minimize these disadvantages for large floor spans, it is known, for example in U.S. Pat. No. 3,712,010, to introduce an upward camber, and hence a positive bending moment, in the decking prior to pouring the concrete floor thereon. This arrangement is intended to counteract the downward deflection and negative bending moment in the decking due to the weight of the concrete floor, to allow a larger floor span to be used without the stress and deflection limits being exceeded. U.S. Pat. No. 3,712,010 discloses two methods of achieving this initial upward camber and positive bending moment. In the first method, embodied as shown in FIGS. 1 to 8 and 13 to 17, there is a tension rod or tendon extending between the ends of the decking. This tension rod is located in an upwardly facing channel of the decking, which is shaped to be symmetrical about a central horizontal plane, the neutral axis of the decking. The tension rod is secured to brackets attached to the ends, or upwardly bent ends, of the decking, so that it is only at the centre of the span that the tension rod is significantly below the neutral axis of the decking. In consequence, the positive bending moment induced in the decking when the tension rod is tightened will be very small, and the stress in the rod has to be substantial to achieve the desired effect, thereby requiring high-grade steel. Furthermore, since the load induced on the ends of the decking through the brackets or bent ends is wholly or largely on the bottom surface of the decking, there will be a negative bending stress induced at the ends of the decking. This further reduces the positive bending stress induced at the centre of the decking span. There is the additional time consuming and costly operation of welding the tension rod to the centre of the decking in the embodiment of FIGS. 5 to 8 and 13 to 17. In the embodiment shown in FIGS. 9 to 12 the tension rod is located in the downwardly facing channel of the decking. Even in this case the tension rod is attached to the decking above the neutral axis (see FIG. 12 in particular), in order to maximize the inclination of the tension rod, generating some negative bending stresses at the ends of the decking as in the above described embodiments. Furthermore, this embodiment introduces the complexity of the centrally disposed post to form the upward camber in the decking, and effectively requires independently applying tension to both ends of the tension rod. The assembly of the post to the decking is a time consuming and costly operation, and exposes the construction to the risk of fire. In addition, this construction may interfere with the passage of services through the floor space. WO 88/01330 discloses a floor channel and tension rod disposed beneath the neutral axis of the channel. However, the neutral axis of the channel is below the central plane, and this low neutral axis, will cause undesirable higher bending stress in the upper horizontal part and lower stresses in the bottom part of the section.
It is an object of the present invention to provide flooring of pre-stressed deck construction that overcomes, at least to a substantial extent, the disadvantages of the known constructions.